Technical Field
The present disclosure relates to an integrated magnetoresistive sensor of an AMR (anisotropic magnetoresistance) type for detecting magnetic fields perpendicular to the chip integrating the magnetoresistive sensor. In particular, the magnetoresistive sensor may be integrated with other magnetoresistors sensitive to magnetic fields parallel to the chip for forming a triaxial magnetometer integrated in a single chip.
Description of the Related Art
AMR-type magnetic-field sensors are used in a plurality of applications and systems, for example in compasses, in ferromagnetic characteristics detecting systems, in detection of currents, and in a wide range of other applications, by virtue of their capacity of detecting natural magnetic fields (for example, the Earth's magnetic field) and magnetic fields generated by electrical components (such as electrical or electronic devices and lines passed by electric currents).
As known, magnetoresistive sensors exploit the capacity of appropriate ferromagnetic materials (referred to as “magnetoresistive materials”, for example, the material known by the term “permalloy” formed by a FeNi alloy) of modifying their own resistance in presence of an external magnetic field.
Integrated magnetoresistive sensors are known having the form of strips of magnetoresistive material arranged on a substrate of semiconductor material, for example silicon. During manufacture, the magnetoresistive material strip is magnetized so as to have a preferential magnetization in a preset direction, referred to as “easy axis” since it is the direction of easier magnetization of the strip, typically the longitudinal direction of the strip.
In the absence of external magnetic fields, the magnetization maintains the set direction, and the strip has a maximum resistance. In presence of external magnetic fields that have a direction different from the preferential magnetization direction, the strip magnetization changes, as well as its resistance, which decreases, as shown in FIGS. 1A and 1B.
In FIG. 1A, a magnetoresistor 1 is formed by a magnetoresistive strip 2 having a longitudinal direction parallel to axis X and forming the easy axis. The magnetoresistor 1 is supplied with a current I flowing in the longitudinal direction of the strip. In the shown condition, in the absence of external magnetic fields, the magnetization M is directed parallel to the easy axis EA.
In FIG. 1B, the magnetoresistor 1 is immersed in an external magnetic field Hy directed parallel to axis Y (referred to as also “hard axis”, i.e., the axis of more difficult magnetization of the magnetoresistive strip 2). In this condition, the external magnetic field Hy causes a rotation of the magnetization M through an angle α with respect to the current I, causing a reduction of the resistance of the magnetoresistive strip 2 according to a law correlated to cos2 α.
In order to linearize the plot of the resistance R at least in an operating portion of the curve, it is further known to form, on the magnetoresistive strip 2, transverse strips 3 (referred to as “barber poles”), of conductive material (for example, aluminum), which are arranged with an inclination of 45° with respect to the direction of easy axis EA, as shown in FIG. 2.
In this situation, the direction of the current I changes, but not the magnetization M (the direction whereof still depends upon the external magnetic field), and the resistance has a linear characteristic around the zero point of the external magnetic field. In this way, possible magnetic fields directed along or having a component parallel to axis Y may be detected easily.
FIG. 3 shows a magnetoresistive sensor including four magnetoresistors 1 of the type illustrated in FIG. 2, connected to form a Wheatstone bridge 4. In the illustrated example, the Wheatstone bridge 4 comprises two magnetoresistors 1a having transverse strips 3 directed at +45° and two magnetoresistors 1b having transverse strips 3 directed at −45°. The magnetoresistors 1a, 1b are arranged in an alternating way in each branch 4a and 4b of the bridge. The two branches 4a, 4b are connected at two input nodes 5, 6 and a biasing voltage Vb is applied across them.
In this way, in the absence of external magnetic field components parallel to the sensing direction (here the field Hx), the output voltage Vo across the output terminals 7, 8 is, to a first approximation, zero. Instead, an external magnetic field Hx causes an increase of the resistivity of two magnetoresistors, for example the magnetoresistors 1a, and a corresponding reduction of the resistivity of the other magnetoresistors, for example the magnetoresistors 1b, causing an unbalancing of the Wheatstone bridge 4 and a non-zero output voltage Vo. Consequently, each variation of resistance due to an external field Hx parallel to the plane of the magnetoresistors 1a, 1b and perpendicular to their extension direction causes a corresponding linear variation of the output voltage Vo.
When it is desired to detect magnetic fields that have components directed along any direction parallel to the main faces of the chip integrating the magnetoresistor (plane XY), it is possible to arrange the magnetoresistors 1 perpendicular to each other, as shown in the sensor 10 of FIG. 4 where, for simplicity, a magnetoresistor 1x, for detecting the component X, and a magnetoresistor 1y, for detecting the component Y, are shown. Obviously, each magnetoresistor 1x, 1y of FIG. 4 may be replaced by a respective Wheatstone bridge similar to that of FIG. 3, wherein the four magnetoresistors 1x are directed perpendicular to the four magnetoresistors 1y. 
By virtue of the high sensitivity of the magnetoresistive sensors of the type referred to above, they have been proposed for use as electronic compasses in navigation systems. In this case, the external field to be detected is represented by the Earth's magnetic field. To a first approximation, the Earth's magnetic field may be considered parallel to the Earth's surface, and reading of the compass may be made using the sensor 10, where X and Y represent the two directions of the plane locally tangential to the Earth's surface. However, since the inclination of the compass with respect to the tangential plane causes reading errors, in order to correct it, it is practical to have three sensors, each sensitive to a respective axis X, Y, Z.
To this end, some compasses integrate the X and Y sensors in a single chip, and the latter is fixed parallel to a base or frame, and the Z sensor, manufactured in a planar way, like the X and Y sensors, in a suitable chip, is fixed to the frame rotated through 90°, in a vertical position. However, in this case, the assembly is complex, and the end device is costly. Further, the packaged device has an excessive volume (in particular an excessive height), which does not enable use thereof in small apparatus.
In order to solve the above problem, a ferromagnetic concentrator has been proposed, arranged alongside a planar magnetoresistor and directed transversely to the sensitivity plane of the magnetoresistor (see, for example, U.S. Patent Publication No. 2013/0299930 and U.S. Patent Publication No. 2014/0159717). For a better understanding, reference may be made to FIG. 5, showing a magnetoresistive sensor 15 formed according to the teachings of above patent application TO2012A001067 in a chip having a substrate 16 of conductive material, for example silicon, and an insulating layer 17. The substrate 16 has a main face 20, which is planar, and the insulating layer 17 houses a magnetoresistor 18, which extends parallel to the main face 20. The magnetoresistor 15 is formed as shown in FIG. 2 and thus comprises a magnetoresistive strip 2 and transverse strips 3 (only one whereof is visible).
A concentrator 21 of soft ferromagnetic material (i.e., one that may be easily magnetized and does not maintain the magnetization after removal of the external magnetic field) extends in a trench 22 in the substrate 17. The concentrator 21 here has a U shape, the arms whereof extend parallel to axis Z and have a length much greater than its thickness. One of the arms of the concentrator 21 extends also in the insulating layer 17, as far as in the proximity or even in contact with the magnetoresistor 15. In an embodiment where the concentrator is in direct electrical contact with the magnetoresistor, to prevent the current flowing in the magnetoresistor from getting lost in the concentrator, the latter is discontinuous.
Consequently, when the magnetoresistive sensor 15 is subject to an external magnetic field Hz directed along axis Z, the arm of the concentrator 21 in contact with the magnetoresistor 18 causes a concentration and deflection of the field lines in horizontal direction (in plane XY) and generation of a horizontal field component Hy directed in the sensing direction. A reading circuit may then detect resistance variations of the magnetoresistor 15 in a known way.
This solution, although enabling detection of magnetic fields perpendicular to the chip with an arrangement of the magnetoresistor parallel to the fixing frame, may undergo improvement.
In fact, to form the concentrator 21 in the substrate 16, it is manufactured prior to forming the magnetoresistor 18 by forming the trench 22 and coating the walls thereof with a thin layer of ferromagnetic material. The step of depositing the ferromagnetic layer is not, however, simple because of the high aspect ratios. Further, in order to form the further structures of the device, the trench is filled with oxide. However, in some cases, the filling operations may entail limitations in treatment temperatures when forming structures after the concentrator, so as to prevent a reduction of the magnetic properties of the concentrator 21.